new in Initializers (PHP 8.4)

Also Known As

TL;DR

Explanation

PHP previously required that default parameter values, attribute arguments, and property initialisers be compile-time constants — scalar values, arrays, class constants, and null. Creating a default object value required a null default and manual assignment in the constructor body. PHP 8.4 allows 'new' expressions in these positions. 'function foo(Logger $log = new NullLogger())' is now valid. The object is created fresh for each call where the default is needed. This also works in property promotion: 'public function __construct(private Logger $log = new NullLogger()) {}' — clean null-object pattern without boilerplate.

Common Misconception

Why It Matters

Common Mistakes

• Using new in initializers in PHP < 8.4 — parse error; check version before adopting.
• Expecting the same instance to be reused across calls — each call that triggers the default creates a new object.
• Using new with classes that have constructor side effects (DB connections, file handles) as defaults — the side effect runs on every call that omits the argument.
• Forgetting this works in attribute arguments too — attribute classes that accept object dependencies can now have clean defaults.

Code Examples

<?php
// ❌ Pre-8.4 null default + manual instantiation boilerplate
class OrderService
{
    private Logger $logger;
    private Cache $cache;

    public function __construct(
        ?Logger $logger = null,
        ?Cache $cache = null
    ) {
        $this->logger = $logger ?? new NullLogger();
        $this->cache  = $cache  ?? new ArrayCache();
    }
}

<?php
// ✅ PHP 8.4 — new in initializers, clean constructor promotion
class OrderService
{
    public function __construct(
        private Logger $logger = new NullLogger(),
        private Cache  $cache  = new ArrayCache(),
    ) {}
}

// Also valid in standalone functions
function sendNotification(
    string $message,
    Mailer $mailer = new SmtpMailer(),
): void {
    $mailer->send($message);
}

References

• ↗ https://www.php.net/releases/8.4/en.php
• ↗ https://wiki.php.net/rfc/new_in_initializers

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